In avalanche photodiodes or photodetectors, incoming light is used to generate carriers (i.e., free electrons or holes). Semiconductor materials are selected for photodiodes based upon the wavelength range of the radiation that is desired to be utilized or detected. Group III-nitride avalanche detectors can presumably be widely functional between 1900 nm and 200 nm (i.e. infrared to ultraviolet radiation). Generally, the binary alloys utilized in such semiconductor devices are Indium Nitride (bandgap of 0.65 eV corresponding to approximately 1900 nm), Gallium Nitride (band gap of 3.4 eV corresponding to approximately 365 nm) and Aluminum Nitride (bandgap of 6.1 eV corresponding to approximately 200 nm). By varying the relative mole fractions of these binaries, ternary or quaternary alloys may be composed that can achieve radiation absorption at intermediate wavelengths to the stated values.
III-Nitride semiconductors are commonly grown in the wurtzite crystal structure and are therefore a polar semiconductor as discussed by Ambacher in O. Ambacher, “Growth and Applications of Group III Nitrides, “J. Phys. D: Appl. Phys. 31 (1998) 2653-2710, herein incorporated by reference as though fully rewritten herein.
U.S. Pat. No. 6,326,654 to Ruden (hereinafter Ruden '654; hereby incorporated by reference) entitled “A Hybrid Ultraviolet Detector,” discloses a semiconductor material avalanche photodiode photodetector. The detector of Ruden '654 is an avalanche photodetector comprised of a group III-nitride semiconductor material, such as aluminum gallium nitride (AlxGa1-xN), serving as a photon to charge carrier transducer, and an avalanche charge carrier multiplication region comprised of different semiconductor materials such as silicon (see abstract).
Deep ultraviolet (DUV) photodetectors sensitive at wavelengths shorter than 260 nm are useful in numerous medical and military applications, including chemical and biological identification and non-line of sight communications. Often, these applications require very low light level or single photon detection and, as a result, photomultiplier tubes (PMTs) are widely used. However, in addition to being large and fragile, photomultiplier tubes require the use of expensive filters to limit the bandwidth of detection. Therefore, a need remains for low cost, compact, high sensitivity devices that offer a narrow and tunable bandwidth. Silicon Carbide (SiC) has emerged as an attractive candidate for DUV pin and avalanche photodetectors (APDs) due to their very low dark currents, small k factor, and high gain. Previously demonstrated APDs exhibit peak quantum efficiency (QE) of 60% at 268 nm and gain values reaching over 1000. Despite this success, SiC is still characterized by poor responsivity below 260 nm and a long absorption tail out to 380 nm. The deep ultraviolet response in pin detectors is hindered by the absorption of high energy photons in the heavily doped surface layer (p- or n-type) where photo-generated carriers are trapped by surface states or recombine due to the short diffusion length of minority carriers. The long wavelength tail results from weak absorption associated with the indirect bandgap of 4H-SiC at 3.23 eV.